In constructing frame structures, walls are vertically supported by wooden studs which are usually 2.times.4's, and floors are supported by wooden beams called joists. The joists are normally 2.times.8's. In standard construction, studs and joists are spaced on 16-inch centers.
The joists are positioned to stand on their two-inch dimension and to span horizontal distances by being supported on their ends. The supports are on outside walls and on internal "bearing" walls. The joists, standing on their narrow edges, are stabilized by having short lengths of joist material nailed between them in a direction perpendicular to their lengths. These short stabilizing pieces are called "blocks" and the process of installing them is called "blocking".
The two-inch and eight-inch dimensions of joists are nominal. Actually those are dimensions before the wood is trimmed. A 2.times.8 is readlly only 11/2 inches thick. Accordingly, when joists are installed on 16-inch centers the open space between them is 14.5 inches.
The process of cutting a block is usually effected by a carpenter measuring 141/2 inches along each edge of the board from which the block is to be cut, drawing a line between those measured distances, and sawing along that line with a power hand saw. A skilled carpenter will be sure that the kerf is in the right place and the saw cut is square with regard to the edges of the block. A properly cut block will be easy to install, yet it will fit snuggly between adjacent joists, and its cut ends will be in contact from the top to the bottom of the joists being supported.
In ordinary construction, a series of joists will support the floor between an outside wall and an interior bearing wall. A second series of joists will support the floor of an adjacent room between that same bearing wall and a second bearing wall or a second outside wall. Above the bearing wall the ends of joists overlap and blocking installed between such lapped joists must be cut 13 inches long if the adjacent joists are on 16 inch centers. Carpenters installing blocking between lapped joists will employ the same technique described above except that the block will be cut 13 inches long.
When a series of joists is installed between two supporting walls, the first joist is 16 inches from the wall that is parallel to the long axes of the joists and each subsequent joist installed is on a 16-inch center from the one adjacent to it. However, the last joist to be installed may not be, and usually is not, exactly 16 inches from the wall that is parallel to it. In such cases, blocking between the final joist in a series and its parallel wall usually will be neither 14.5 inches nor 13 inches in length, but rather whatever length is needed to span the distance between the final joist and its parallel wall.